This invention relates in general to electrical circuits, and more specifically to a voice activated circuit and a radio using said circuit.
A voice activated switch (VAS) or voice operated transmit (VOX) circuit in the specific case of a radio, is required for hands-free operation of an electronic device (e.g., two way radio, tape recorder, etc.). A VOX circuit allows a radio user to activate the radio""s transmitter without the need to activate the Push-to-Talk (PTT) switch on the radio. The radio transmitter is activated whenever the radio user speaks into the radio""s microphone. A traditional VAS circuit only estimates energy in the audio band so that it is unable to distinguish between voice and noise in the incoming signal.
An ideal radio VOX circuit should detect the instant a speaker commences to talk and immediately generate a control signal to activate the radio""s transmitter. In reality however, a delay exists in both the speech detection and the amount of time it takes to activate the transmitter. The main focus of VOX circuit design is essentially placed on detecting speech accurately and minimizing process delays.
A simple prior art VOX circuit estimates energy in the 300-hertz (Hz) to 3,000 (kilohertz, or kHz) audio band in order to determine whether or not to activate the transmitter. This type of VOX circuit is simple but makes no judgment of whether the energy within the audio band is from someone attempting to talk to the radio, a car horn, or a white noise. This of course can cause the radio transmitter to become activated because a sound in the audio band is present (e.g., noisy environments, etc.).
Other more sophisticated VOX approaches, such as those using fast-fourier transforms (FFT), cepstrum, time-frequency representations, Linear Prediction Coding (LPC), Hidden Markov Model (HMM), etc. introduce either significant hardware complexity, high software computing power requirements, or both. These types of sophisticated and more expensive VOX circuits may also not be appropriate for low cost radio designs. A need thus exists in the art for a VOX circuit that can provide for improved voice detection while at the same time maintaining a fairly simple and low cost design.